We describe sexual dimorphism in coloration and the reproductive cycle in Tropidurus semitaeniatus, a widely distributed lizard species in the caatinga of northeastern Brazil. Yellow and yellow-and-black patches occurred on the ventral surface of the thighs and precloacal flap in 15 and 11 adult males, respectively. Thirteen reproductively active males (collected November to March) had ventral patches with intense pigmentation, contrasting with the faded pigmentation of these patches in two males during the maturation stage (collected in October) and in 11 individuals in regression phase (collected April to September). Thirty-six adult females, reproductively active and not, and 21 juveniles lacked ventral color patches. There was a significant relationship between snout–vent length and testis volume during maturation and regression stages in males with both types of color patches. We conclude that ventral color patches displayed by male T. semitaeniatus are closely associated with their reproductive cycles.